Update QHighDpiScaling on DPI change

Make sure Qt reacts correctly to DPI changes while the
application is running, also when going from “standard-dpi”
to “high-dpi” (like Windows 100% to 200%).

Call QHighDpiScaling::upateHighDpiScaling() on DPI
change and update the m_usePixelDensity flag from there.

Fixes: QTBUG-85384
Pick-to: 5.15
Change-Id: I8ca83e4eea76cc8ba701a18e1f8c535b9953918f
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/gui/kernel/qguiapplication.cpp

@@ -3092,6 +3092,8 @@ void QGuiApplicationPrivate::processScreenLogicalDotsPerInchChange(QWindowSystem
     if (QCoreApplication::startingUp())
         return;
 
+    QHighDpiScaling::updateHighDpiScaling();
+
     if (!e->screen)
         return;
 

src/gui/kernel/qhighdpiscaling.cpp

@@ -495,6 +495,8 @@ void QHighDpiScaling::updateHighDpiScaling()
     if (QCoreApplication::testAttribute(Qt::AA_DisableHighDpiScaling))
         return;
 
+    m_usePixelDensity = usePixelDensity();
+
     if (m_usePixelDensity && !m_pixelDensityScalingActive) {
         const auto screens = QGuiApplication::screens();
         for (QScreen *screen : screens) {